Heretofore, there has been an acceleration sensor to detect acceleration, by which the acceleration is measured as follows: a piezoelectric element loaded with a weight is mounted at the bottom of a metal housing; an external acceleration is exerted to the weight to generate an inertial force; a stress is applied to the piezoelectric element by the inertial force of the weight, and a voltage is generated by the piezoelectric element corresponding to the applied stress; and the voltage is converted to an acceleration (for example, Patent Document 1).
Further, a multiaxial acceleration sensor has been also manufactured, in which piezoelectric elements each loaded with a weight as described above are arranged so that their respective detecting acceleration directions are perpendicular to one another, so that accelerations can be detected in all of three dimensional directions x, y and z (for example, Patent Document 2).
Such an acceleration sensor uses a so-called piezoelectric bimorph, which is made of rectangular piezoelectric ceramic plates, each of which is metalized by having metal films with conductivity attached on both sides and which are bonded together directly or with a metal plate interposed therebetween so that their respective flat surfaces face each other.
In the acceleration sensor, one of the ends in the length direction of the piezoelectric bimorph is fixed to e.g. a frame, and the other end is permitted to be a free end.
In such a state, when an acceleration is exerted to the piezoelectric bimorph in the thickness direction (direction perpendicular to the flat surface being a main surface of the piezoelectric ceramic plate), the piezoelectric bimorph is deflected, and a voltage corresponding to the amount of the deflection is generated at the electrodes on both sides of the piezoelectric bimorph. Then, depending on the magnitude of the voltage, the acceleration exerted to the piezoelectric bimorph is detected.
A construction in which such piezoelectric bimorphs are fixed so that the directions of the main surfaces of the bimorphs are perpendicular to each other (for example, Patent Document 3) or an acceleration sensor in which a single sensor element, such as a piezoelectric bimorph, is mounted on a circuit board so that it turns to any optional direction, is manufactured. There is also a construction in which a plurality of such acceleration sensors are used in combination to enable detection of accelerations in various directions (for example, Patent Document 4).
Further, there is also a construction in which, using an inertial mass plate which changes its position relative to a counter electrode, an acceleration exerted to the inertial mass plate is detected based on a change in position of the inertial mass plate i.e. based on a change in capacitance between the counter electrode and the inertial mass plate (for example, Patent Document 5).
On the other hand, in recent years, attention has been paid to a so-called wireless sensor network, in which sensors distributed at multiple sites are interconnected by the wireless communication technology.
In the wireless network system, sensor nodes which have a sensor circuit to convert a physical amount detected by each sensor into an electric signal and a wireless circuit having data storage and diagnostic functions as well as transmit-receive functions, are networked wirelessly.
With topologies, network control function and security function provided by this wireless network, an extremely large scale and wide area sensing system can be easily constructed.
And, such a wireless network system may be utilized for monitoring of equipments in a large chemical plant, maintenance of structures such as roads, bridges and dams, prediction of landslides, and so on.
A wireless sensor network takes advantage of characteristics of wireless, such as convenience, extendibility, and easy installation at a site where cabling is difficult.
Accordingly, it is not possible to supply power through lines to the respective sensor nodes. Therefore, generally, each node needs to have a built-in battery as a power source to actuate the functions of e.g. internal circuits.
Also, in many cases, the sensor nodes are installed at sites where it is difficult to replace batteries, considering the usage of the sensor nodes. Accordingly, prolongation of battery life remains to be a problem to be solved.
Under the circumstances, as one of the means to reduce power consumption, it has been proposed to control the communication timing by e.g. synchronization of wireless communications (for example, Chapter 4 of Non-patent Document 1).
As another means to reduce power consumption, a method of carrying out a flexible intermittent operating control over sensing by sensor circuits or timing of wireless communications has been proposed (for example, Patent Document 6).
Patent Document 1: JP A 64-41865
Patent Document 2: JP U 1-112468
Patent Document 3: JP A 3-156375
Patent Document 4: JP A 08-15302
Patent Document 5: JP A 2007-333618
Patent Document 6: JP A 2008-28499
Non-patent Document 1: Chapter 3 “Vision for the future of the ubiquitous sensor network” in “The final report for a realization of the ubiquitous sensor network”, Ministry of Internal Affairs and Communications, September 2004 (http://www.soumu.go.jp/s-news/2004/040806—4_b2.html, accessed on Mar. 6, 2008)